Gate valves have been commonly used for many years and have been employed in a variety of services such as in the oil and/or gas services of the petroleum industry. However, such valves have been designed for service involving pressures of only several thousand pounds per square inch. Today due to the depleted energy supply it has become necessary to explore and produce oil and gas under very high pressures of the range 30,000 psi (pounds per square inch) or more. Such high pressures have required greater sealing capacity and better safety features in such valves.
Prior art gate valves generally consist of a valve body having inlet and outlet conduits with a chamber there-between. A gate having an opening therethrough is disposed in a valve seat mounted within the chamber and the gate is mounted on a stem. The chamber is closed by a bonnet affixed to the valve housing. The stem reciprocates within the bonnet and is attached to a reciprocating apparatus for moving the gate between a closed position in which flow is blocked through the conduits and an open position in which flow is allowed through the conduits and the gate opening. Such a valve is illustrated in U.S. Pat. Nos. 3,538,938 and 3,696,831.
It is also well known to seal between the valve seat and the valve housing by using an o-ring as shown in U.S. Pat. Nos. 2,957,492 and 3,348,567 and non extrusion seals as shown in U.S. Pat. No. 4,264,054. Further in automatic pressure sealed gate valves shown in U.S. Pat. Nos. 3,095,604; 3,538,938; and 3,696,831, fluent sealant rings are disposed around the valve seats to provide sealing between the valve housing and valve seats in response to line pressure on fluent sealant reservoirs in communication with the fluent sealant rings, and o-rings are installed on each side of the fluent sealant rings to isolate the fluent sealant in the fluent sealant rings from contamination by line fluids. This same principle is used for seals in wellheads as shown in U.S. Pat. No. 2,952,479. It is also known to locate an o-ring seal on the end of the gate seat as shown in U.S. Pat. No. 3,696,831. U.S. Pat. Nos. 3,057,630 and 3,758,072 teach combining an elastomeric O-ring and a polyflouroethylene polymer such as the brand "Teflon" or "nylon" or other low-friction plastic ring to make a seal ring. Further U.S. Pat. No. 3,103,366 teaches a deformable metal ring, as for example of aluminum, with an inner deformable plastic ring of polytetrafluororoethylene whereby the metal ring can adapt itself to slightly varying recess clearances and prevent the plastic ring from extruding through the clearances under pressure.
Two piece seats are also known in the art for floating seats as shown by brochures of Model 20 gate valves of FMC, and for replaceable seals through non metallic inserts to seats of Seaboard as shown by its brochure and U.S. Pat Nos. 4,376,526 and 4,124,194. Another replaceable seal in seats is shown by U.S. Pat. No. 4,163,544. A floating seat with a replaceable seal support element is shown by U.S. Pat. No. 3,273,855.
Secondary seals where the gate forms a metal-to-metal seal with the seat and where a resilient seal is moved toward the gate when the gate is in the closed position to form a secondary fluid-tight seal with the gate is shown by U.S. Pat. No. 4,377,273. Manual operations for removal of flow blocking plates and primary seal actuation after such removal are shown in U.S. Pat. No. 2,203,989. Sliding seals are shown in U.S. Pat. No. 2,471,941. Other valve seals are shown by U.S. Pat. Nos. 3,472,270, 2,134,277, 2,776,813, 1,692,496 and 3,557,822.
As discussed in U.S. Pat. No. 4,377,273, one problem frequently encountered in the case of gate valves is the lack of formation of adequate seals between the gate or gates and the seat or seats. In turn, if a seal is inadequate, the gate valves are often prone to leak through the seal. In U.S. Pat. No. 4,377,273, an invention is disclosed which seeks to provide an adequate seal for gate valves in the form of a positive bidirectional cam actuated and deactuated resilient secondary fluid seal.
While the invention described in U.S. Pat. No. 4,377,273 is believed to be advantageous for many applications, it has been found that there are applications in which other secondary seals may be provided. In particular, in high pressure situations wherein the formation of the secondary seal causes a significant pressure differential on the two sides of the gate and wherein the secondary seal is located on the downstream side, it has been found that for some applications, the drag on the secondary seal caused by movement of the gate to open when the secondary seal is fully effected may exaggerate the wear of the secondary seal. The additional drag on the gate also increases the effort required to open the valve. It is desirable in those instances to be able to deenergize or release the secondary seal prior to substantial linear movement of the gate across the seal.
In this regard, it is not necessary to totally withdraw the seal from the gate, but rather it is desirable to relieve the full pressure of the seal against the gate, or "deenergize" the seal, such that the pressure of the fluid moving through the valve will flow between the seal and the gate to preclude wear.
Additionally, in some high pressure situations wherein the secondary seal is fully effected or energized with no pressure in the valve body, and pressure is thereafter applied from the upstream side, there is no way for the fluid to enter into the body without leaking past the energized upstream secondary seal. Since the upstream secondary seal is mechanically energized, it is maintained until the pressure load of the fluid overcomes the mechanical load. In some instances, the pressure of the fluid is great enough to blow past the seal once it overcomes the mechanical energy of the seal, thereby damaging the resilient sealing ring contained in the seal.
It is therefore desirable to provide a pressure relief means from a secondary seal subjected to such pressurized fluid under the above conditions, in which the fluid will force the seal away from the gate and allow movement of the fluid past the seal without damaging the resilient seal element when the pressure of the fluid reaches a selected value.
Also, seats that are press fitted into the valve body are difficult to replace, particularly in the field. Special tools are required to overcome the press fit and the valve body counterbore is subject to damage by gaulling between the seat OD and valve body counterbore during installation and removal of the seat. It is desirable to provide a seat that can be replaced easily without disturbing the press fit between the seat and the body.